Magnetic resonance imaging (MRI) involves the transmission and receipt of radio frequency (RF) energy. RF energy may be transmitted by a coil. Resulting magnetic resonance (MR) signals may also be received by a coil. Conventionally, MR coils have had a set of elements that included a long copper trace arranged in a loop in which an electric current could be induced by nuclear magnetic resonance (NMR) signals produced by an object near the loop. The set of elements also included other items including capacitors, resistors, pre-amplifiers, a PIN diode, or additional signal processing elements. Conventionally, each coil had all of the elements. Some minimalist coils have been designed where some RF coil elements are removed from the coil. These minimalist coils have been used in MR procedures where a patient may be moved during the procedure.
An MRI procedure may employ several coils. In a conventional situation, even when each of the several coils had all of the conventional RF coil elements, only selected coils (e.g., coils in the excitation zone produced by the MRI apparatus) were in use at any given time. Even though only some coils were in use at any time, each coil needed to be manually plugged into an apparatus that would allow the selective use of a coil as it was needed. Plugging in the coils and checking that they were attached to the apparatus correctly consumed operator time and produced opportunities for errors. The operator time occurred while the patient was lying on the MR apparatus waiting for their scan. Also, plugging in and then unplugging the connectors produced wear and tear that may have led to the connectors breaking or needing servicing, which may have led to down time for the MRI apparatus.
United States patent application 2009/0315556 ('556) is incorporated herein by reference. In conventional coils, the inductors, capacitors, resistors, PIN diodes, or other circuit elements were all located on the MR coil. In the '556, some of these elements were moved off the MR coil and were only accessed via a coupling interface. Unfortunately, conditions at the coupling point may have created varying capacitance for the MR coil, which may have had negative impacts on coil use.
'556 describes a “contacting system” for bringing MR local coils in contact with MR coil circuitry that was moved off the MR coil. The contacting system has “a number of coil coupler elements that are electronically connected with the MR local coils and apparatus coupler elements that are mounted on the MR scanner housing and that are electrically connected with the unit for additional signal processing. '556 describes how “the coil coupler elements and the apparatus coupler elements are fashioned so that given a movement of the local coils in the MR scanner a successive contacting of at least a portion of the coil coupler elements with apparatus coupler elements ensues at least over a specific path segment.” For example, as a table holding a patient moves in and out of an MR apparatus, the coil coupler elements of different local coils come in contact with the apparatus coupler elements to complete a circuit that allows signals received in the MR local coils to be passed to the circuits that were moved to the apparatus. While the '556 facilitates having a minimalist MR coil by moving MR coil circuitry off the MR coil, the '556 also produces a new problem that did not exist before the MR coil coupler element to apparatus coil coupler element approach was used.
The '556 identifies the problem in [0055], which reads “the signal transmission is adulterated by a variation of the capacitance.” The capacitance of an MR coil affects the performance of the MR coil. Recall that an imaging coil needs to be able to resonate at a carefully selected Larmor frequency. Imaging coils include inductive elements and capacitive elements. The resonant frequency, v, of an RF coil is determined by the inductance (L) and capacitance (C) of the inductor capacitor circuit according to:
  v  =      1          2      ⁢              ∏                  LC                    
The '556 moves some elements that are involved in tuning from an MR coil to a separate apparatus and then selectively couples the simpler MR coil to the additional elements on an as-needed basis using couplers on both the MR coil and the separate apparatus. The '556 allows coupling of minimalist MR coils to additional signal processing elements in a fashion that facilitates continuous movement of a subject positioning device (e.g., table on which patient is lying in MR scanner). In the '556, some of the L and C may occur due to elements on the local MR local coil, some of the L and C may occur due to elements that were moved off the local MR coil, and some of the L and C may occur at the coupling point between the MR coil coupling elements and the apparatus coupling elements. Due to the relative motion of the coupling elements, the capacitance at the coupling point may vary. Varying capacitance produces a sub-optimal tuning situation.
Imaging coils may need to be tuned. Tuning an imaging coil may include varying the performance of a capacitor. Recall that frequency: f=ω/(2π), wavelength: λ=c/f, and λ=4.7 m at 1.5 T. Recall also that the Larmor frequency: f0=γB0/(2π), where γ/(2π)=42.58 MHz/T; at 1.5 T, f0=63.87 MHz; at 3 T, f0=127.73 MHz; at 7 T, f0=298.06 MHz. Basic circuit design principles include the fact that capacitors add in parallel (impedance 1/(jCω)) and inductors add in series (impedance jLω). These equations and relationships aren't just convenient descriptions for MR coils; they are design requirements for being able to produce coils that actually receive NMR signals.
Referring to Related Art FIG. 1, a single RF coil segment 102 is shown schematically to include an inductance 103, a resistance 104, and a capacitance 105. Capacitance 105 is selected and controlled to tune the segment 102 to a desired frequency (e.g., Larmor frequency). The RF coil segment 102 is connected across the output of a current control circuit 106 that is driven by an input signal 107 to produce a current in the RF coil segment 102. In the '556, some of the elements illustrated for coil 102 may remain on the minimalist coil while some of the elements may be moved off the coil (e.g., to the MR apparatus). The minimalist coil may then be connected using the coil coupling elements and apparatus coupling elements described in the '556. Unfortunately, coupling approaches and designs like those described in the '556 and elsewhere may unacceptably vary the capacitance of coil segment 102 and thus negatively impact tuning.
There are many design issues associated with MRI RF coil design. For example, the inductance of a conventional coil depends on the geometry of the coil. For a square coil with a side length a and wire diameter f: L=[μ0/π] [−4a+2a √2−2a log(1+√2)+2a log(4a/f)]. For a loop coil with loop diameter d and wire diameter f: L=[μ0d/2] [log(8d/f)−2]. Thus, the selection of the geometry of a coil determines, at least in part, the inductance of the coil.
The resistance of a coil also depends on the geometry of the coil. The resistance R of a conductor of length l and cross-sectional area A is R=ρl/A, where ρ is the conductor resistivity and is a property of the conductor material and the temperature. For a copper wire coil with loop diameter d and wire diameter f: R=dρCu/(fδCu). For a copper foil coil with loop diameter d, copper thickness t, and copper width w: R=πdρCu/(2wδCu), where t is much greater than the copper skin depth and w is much greater than t. Thus, the selection of the geometry of a coil and the material (e.g., wire, foil) determines, at least in part, the inductance of the coil. The length of the loop also impacts the properties of the coil.
Coils may be characterized by their signal voltage, which is the electro-motive force (emf) induced in a coil: ξ=−∂φ/∂t∝=−∂(B1·M0)∂t, where φ is the magnetic flux across the coil (closed loop), magnetization M0=Nγ2(h/(2π))2s(s+1)B0/(3kBTS)=σ0B0/μ0, where N is the number of nuclear spins s per unit volume (s=½ for protons) and Ts is the temperature of the sample. Since ω0=γB0, ξ∝ω02. The noise in a coil may be thermal (e.g., v=(4kBTSRΔf)1/2, where R is the total resistance and Δf is the bandwidth of the received signal). The signal to noise ratio (SNR) for a coil may be described by ξ/v.
Coils may be used for transmitting RF energy that is intended to cause NMR in a sample. The frequency at which NMR will be created depends on the magnetic field present in the sample. Both the main magnetic field B0 produced by the MRI apparatus and the additional magnetic field B1 produced by a coil contribute to the magnetic field present in the sample. For a circular loop coil, the transmit B1 field equals the coil sensitivity. A circular loop of radius a carrying a current I produces on axis the field: B=μ0 I a2/[2(a2+z2)3/2].
RF coils for MRI may need to be tuned and matched. Tuning involves establishing or manipulating the capacitance in a coil so that a desired resistance is produced. Matching involves establishing or manipulating the capacitance/inductance at a feeding point of a coil so that a desired impedance (e.g., 50 Ohm) is achieved. If the capacitance at a coupling point between a minimalist RF coil and circuitry with off-coil elements varies, then tuning and matching may be complicated. When tuning, the impedance z may be described by Z=R+jX=1/(1/(r+jLω)+jCω). Tuning may be performed to achieve a desired tuning frequency for a coil. ω0 identifies the desired tuning frequency. ω0, may be, for example, 63.87 MHz at 1.5 T. The size of a conventional coil facilitates estimating inductance L. With an estimate of L available, values for capacitors can be computed to produce a desired resonant peak in an appropriate location with respect to ω0. Once capacitors are selected, the resonant peak can be observed and a more accurate L can be computed. The capacitors or inductor at the feeding point can then be adjusted to produce the desired resistance for matching.
Conventional coils may use PIN diodes. When forward-biased, a PIN diode may produce a negligible resistance (e.g., 0.5Ω), which is essentially a short-circuit. When reverse-biased, a PIN diode may produce a high resistance (e.g., 200 kΩ) in parallel with a low capacitance (e.g., ˜2 pF), which is essentially an open-circuit.
Related Art FIG. 2 illustrates a schematic of a simple conventional RF coil 200 for MRI. The coil 200 is illustrated as a loop 210. Loop 210 has elements that produce a resistance (R) (e.g., resistor 220) and that produce an inductance (L) (e.g., inductor 230). A conventional loop may include a matching capacitor 240 and tuning capacitor 250 that produce capacitance (C). The simple RF coil 200 may be referred to as an LC coil or as an RLC coil. Conventionally, the resistor 220, inductor 230, and capacitor 250 may all have been two terminal passive elements that were soldered to copper wire or copper foil that was attached to a printed circuit board. The resistor 220, inductor 230, and capacitor 250 may all have been on the RF coil 200. Example minimalist coils may move part of resistor 220, inductor 230, or capacitor 250 to off-coil circuitry with which a selective coupling is made. Variable capacitance in the selective coupling may negatively impact the performance of coil 200.
A resistor may be, for example, a passive, two-terminal electrical component that implements electrical resistance as a circuit element. Resistors reduce current flow. Resistors may have fixed resistances or variable resistances. The current that flows through a resistor is directly proportional to the voltage applied across the resistor's terminals. This relationship is represented by Ohm's Law: V=IR, where I is the current through the conductor, V is the potential difference across the conductor, and R is the resistance of the conductor.
An inductor, which may also be referred to as a coil or reactor, may be a passive two-terminal electrical component that resists changes in electric current. An inductor may be made from, for example, a wire that is wound into a coil. When a current flows through the inductor, energy may be stored temporarily in a magnetic field in the coil. When the current flowing through the inductor changes, the time-varying magnetic field induces a voltage in the inductor. The voltage will be induced according to Faraday's law and thus may oppose the change in current that created the voltage.
A capacitor may be a passive, two-terminal electrical component that is used to store energy. The energy may be stored electrostatically in an electric field. Although there are many types of practical capacitors, capacitors tend to contain at least two electrical conductors that are separated by a dielectric. The conductors may be, for example, plates and the dielectric may be, for example, an insulator. The conductors may be, for example, thin films of metal, aluminum foil or other materials. The non-conducting dielectric increases the capacitor's charge capacity. The dielectric may be, for example, glass, ceramic, plastic film, air, paper, mica, or other materials. Unlike a resistor, a capacitor does not dissipate energy. Instead, a capacitor stores energy in the form of an electrostatic field between its conductors.
When there is a potential difference across the conductors, an electric field may develop across the dielectric. The electric field may cause positive charge (+Q) to collect on one conductor and negative charge (−Q) to collect on the other conductor.
Related Art FIG. 3 illustrates a schematic of another simple RF coil 300 for MRI. RF coil 300 may also be referred to as an LC coil or as an RLC coil. The coil 300 is illustrated as a square loop 310. Loop 310 has elements that produce a resistance (e.g., resistor 320) and that produce an inductance (e.g., inductor 330). A conventional loop may include a capacitor 340 and capacitor 350 that work together to achieve matching. Once again, the resistor 320, inductor 330, and capacitors 340 and 350 may have been soldered to copper wire or copper foil that was attached to a printed circuit board. Coil 300 is contrasted with coil 200 (Related Art FIG. 2) that included capacitor 250 for tuning purposes. When the resistors, inductors, capacitors, or other circuit elements are located across an interface between a coil coupling element and an apparatus coupling element that produces varying capacitance, tuning even simple RF coil 300 may be difficult, if even possible at all.
Coils in close proximity to each other may need to be decoupled from each other. Coils may also need to be decoupled when not in use. Related Art FIG. 4 illustrates a conventional RLC coil 400 that performs decoupling using components L31 and D31. Coil 400 includes capacitors C31, C32, C33, and inductors L31, L32, and L33. Coil 400 includes a pre-amplifier circuit 410. Coil 400 also includes a PIN diode D31. Recall that a PIN diode has a wide, lightly doped near intrinsic semiconductor region positioned between a p-type semiconductor region and an n-type semiconductor region that are used for Ohmic contacts. The wide intrinsic region makes the PIN diode suitable for fast switches. Fast switching may be employed in MRI coils. In transmit mode, the PIN diode D1 may be turned off (e.g., shorted).
In conventional coil 400, a single capacitor C32 is illustrated to represent one or more capacitors that may be employed in the coil 400. Thus, capacitor C32 may be an equivalent capacitor of multiple breaking point capacitors that may appear in coil 400 minus capacitor C31. Inductor L32 represents the inductance of the coil. The inductance may be produced, for example, by a copper trace that forms the coil 400.
In the conventional coil 400, capacitor C31 is the breaking point capacitor that is used for decoupling the coil 400 from other MRI coils. Capacitor C31 and inductor L31 are in parallel resonance and the impedance across capacitor C31 is high. Capacitor C31 is a single high impedance point in coil 400. Since the impedance across capacitor C31 is high, an induced voltage on coil 400 cannot generate a large current through capacitor C32. In a conventional coil like coil 400, all the circuit elements are located on the coil. In a coil like that described in the '556, some of the circuit elements are removed from the coil and become part of the coil circuit only when the coil 400 is connected to the elements by the coil coupling element to apparatus coupling element interface. The interface may produce varying capacitance which may negatively impact tuning or decoupling of coil 400.
U.S. Pat. No. 7,602,182 ('182) is also incorporated herein by reference. The '182 describes a local coil that is operable in combination with the magnet system of an MR apparatus to generate MR signals in an examination subject and to detect MR signals resulting therefrom. The local coil is a more minimalist coil that has had some coil elements (e.g., pre-amplifier) moved off the coil and into the MR apparatus. The '182 differs from conventional systems because the local coils do not each have to be plugged in to the MR apparatus but rather are contacted on an as-needed basis as the patient is slid in or out of the bore. The '182 describes how “a local coil can be automatically coupled with an evaluation device when it is located in the excitation region and is otherwise caused to be decoupled from the evaluation device.” [0017] The evaluation device refers to the MR apparatus. In the '182, the local coil is coupled to the apparatus by a “patient bed coupling element” and a “base body coupling element.” These elements correspond to the coil coupling element and the apparatus coupling element of the '556. The interface between the patient bed coupling element and the base body coupling element may be galvanic, capacitive, or inductive. The interface between the patient bed coupling element and the base body coupling element may produce a variable capacitance that may negatively impact tuning or decoupling of the local coil.
The '182 describes benefits of having a minimalist coil that can be selectively coupled to circuit elements that conventionally resided on the coil. For example, the '182 describes having shorter cables that reduce attenuation and having one set of circuit elements (e.g., pre-amplifier, PIN diode) in the apparatus rather than having one set of circuit elements per coil. The '182 also describes not having to plug each coil into the evaluation unit but rather having the coils that are in the excitation region of the evaluation unit automatically connected as the patient bed moves in the bore.
The '182 describes the varying coupling that occurs as the coil coupling elements move with respect to the apparatus coupling elements. For example, the '182 describes how “a degree of coupling k1 of the first patient bed coupling element 9 with the first base body coupling element 11 is thus zero in this travel position. When the patient bed 5 is now moved further in the travel direction z, the degree of coupling k1 with which the first patient bed coupling element 9 couples with the first base body coupling element 11 increases gradually toward a maximum value. This state is reached when the first patient bed coupling element 9 and the first base body coupling element 11 are situated precisely opposite one another . . . . After this the degree of coupling k1 gradually decreases again to zero.” [0073]-[0074] This variable coupling may produce conditions where capacitance may vary, which may in turn have negative implications for tuning or decoupling.
The '182 also touches on tuning a coil. See, for example, [0080]-[0082]. Unfortunately, the '182 also describes how “the patient bed coupling elements 9 are in all cases designed identically among one another” and how “the base body coupling elements 11 are also in all cases designed identically.” In [0099] the '182 describes how “the coupling elements 9, 11 . . . are advantageously respectively fashioned as a pair of narrow coupling strips 34” and how “the coupling strips 34 of each coupling element 9, 11 are thereby advantageously adjacent to one another at their narrow sides to minimize the unavoidable parasitic capacitance between them.” Thus, the '182 may experience the same varying capacitance issue as the '556.
According to an aspect of the present technique, a system including a coil coupler and a minimalist magnetic resonance radio frequency (MMRRF) coil is presented. The coil coupler is electrically connected to the MMRRF coil. The coil coupler being a capacitive plate. The system also includes a plurality of off coil circuitry couplers (OCCCs) electrically connected to a corresponding plurality of off coil MR circuits (OCMRC) in a one-to-one manner. The OCCC is a capacitive plate. In the system the coil coupler is moveable relative to the plurality of OCCCs by a magnetic resonance (MR) apparatus during an MR procedure. In the system, the coil coupler and the plurality of OCCCs are arranged so that the coil coupler may be in capacitive contact with N OCCCs at a time, N being an integer greater than or equal to zero. In the system, the coil coupler and the plurality of OCCCs are arranged so that one OCCC can only be in capacitive contact with at most one coil coupler at a time, and where when the coil coupler is in capacitive contact with an OCCC the coil coupler is disposed parallel to the OCCC. Furthermore in the system, when the coil coupler is in capacitive contact with a selected OCCC the MMRRF coil associated with the coil coupler is electrically connected to a selected member of the plurality of OCMRC associated with the selected OCCC, where being electrically connected to the selected member allows MR signals detected by the MMRRF coil to flow from the MMRRF coil to the selected member. In the system, when the coil coupler is in capacitive contact with one or more OCCCs and while the coil coupler is moving relative to the plurality of OCCCs, there is a constant capacitance between the coil coupler and the one or more OCCCs to which the coil coupler is in capacitive contact in the locations where the coil coupler is in capacitive contact with the one or more OCCCs, and where the constant capacitance remains within a desired range for the MMRRF coil causing the MMRRF coil to remain tuned to within a desired frequency range.
According to another aspect of the present technique, a system that includes a magnetic resonance (MR) apparatus, two or more MR coils, and two or more MR coil circuits is presented. The MR apparatus moves a patient through an excitation zone during an MR procedure. The two or more MR coils include a loop and a coil connector. The two or more MR coils are moved through the excitation zone during the MR procedure by the MR apparatus. At least one of the two or more MR coils acquires MR signals while in the excitation zone. Each MR coil circuit includes a signal processing circuit and a circuit connector. In the system, when a member of the two or more MR coils is in the excitation zone, the coil connector associated with the member of the two or more MR coils creates an electrical connection to N members of the two or more MR coil circuits via the coil connector associated with the member of the two or more MR coils and the circuit connectors associated with the N members of the two or more MR coil circuits, where N is an integer greater than or equal to one. In the system, the electrical connection allows MR signals detected in the member of the two or more MR coils to flow to the N MR coil circuits, and the electrical connection produces a constant capacitance for the member of the two or more MR coils as it moves through the excitation zone and is connected to different members of the N MR coil circuits.
According to another aspect of the present technique, a magnetic resonance (MR) apparatus is presented. The MR apparatus includes an MR data acquisition unit that acquires MR data from an examination subject interacting with the MR data acquisition unit. The MR data acquisition unit includes a local coil that participates in the acquisition of the MR data by transmitting radio frequency (RF) signals into the examination subject and by receiving resulting MR signals from the examination subject. The local coil has a coil coupler element electrically connected thereto and mounted in the MR data acquisition unit. In the MR data acquisition unit is a signal processor located remote from the local coil. A plurality of apparatus coupler elements mounted in the MR data acquisition unit are electrically connected to the signal processing unit. The MR apparatus further includes a moveable element on which the local coil is carried. The moveable element moves in a movement path in the MR data acquisition unit causing the coil coupler element to be in contact with ‘N’ of the plurality of apparatus coupler elements at a time, ‘N’ being an integer greater than or equal to zero. The coil coupler element is disposed on the moveable element and the plurality of apparatus coupler elements are disposed on the MR data acquisition unit to cause the coil coupler element to couple to N members of the apparatus coupler elements within at least a segment of the movement path. The coil coupler element and the plurality of apparatus coupler elements have shapes that produce a constant capacitance between the coil coupler element and the N apparatus coupler elements as the coil coupler element moves along the movement path, and producing the constant capacitance for the local coil while the local coil is coupled to members of the plurality of apparatus coupler elements.
According to yet another aspect of the present technique, a magnetic resonance (MR) system is presented. The MR system includes a base body having a magnet system that generates magnetic fields in an examination region of the base body. In the MR system is a patient bed that is movable in a travel direction through a travel region relative to the base body, the patient bed being configured to receive an examination subject thereon to move the examination subject through the examination region. The MR system also includes an antenna arrangement operable in combination with the magnet system to interact with the examination subject to generate MR signals in and receive MR signals from the examination subject, the antenna arrangement having a local coil configured to at least detect the MR signals. Furthermore the MR system includes an evaluation device that evaluates the MR signals detected by the local coil. In the MR system are a plurality of base body coupling elements and one or more patient body coupling elements. The plurality of base body coupling elements are connected to the evaluation device, the base body coupling elements being located at predetermined base body locations. Each of the patient bed coupling element is connected to the local coil, and is located at a predetermined patient bed location at the patient bed. In the MR system the plurality of base body coupling elements and the patient bed coupling element are respectively configured and located to couple one patient bed coupling element with N of the base body coupling elements as the local coil travels through the travel region in the travel direction to feed the MR signals detected by the local coil from the local coil via the patient bed coupling element and the base body coupling elements to the evaluation device, N being an integer greater than or equal to zero. Furthermore, the plurality of base body coupling elements and the patient bed coupling element are configured to produce a constant capacitance in the local coil as the local coil moves through the travel region in the travel direction.